Serum electrolytes play a critical role in regulating normal physiologic functioning within and between cells. The testing of serum electrolytes is one of the most common analytical tests performed within hospitals. Such measurements are employed for routine monitoring of a patient as well as in emergency and life-threatening situations. Because of the vital role of electrolytes in normal physiologic responses, it is important that the measurement of the serum levels of electrolytes can be performed efficiently, accurately, and inexpensively.
Potassium is one electrolyte important in the regulation of intracellular enzymatic function and in proper electrical functioning of excitable tissues, e.g., the heart, brain, and muscle. Serum potassium is normally maintained within the narrow range of 3.5 to 5.5 mEq/1. The intracellular/extracellular potassium ratio (Ki/Ke) largely determines neuromuscular tissue excitability. Because most of the potassium ions are intracellular, neuromuscular excitability is markedly affected by small changes in extracellular, e.g., serum, potassium levels. For example, even small changes in potassium serum levels can cause abnormal cardiac arrhythmias, affecting cardiac function. Therefore, the monitoring of potassium serum levels provides useful information on a variety of diseases and disorders, including nephropathy (e.g., acute renal failure, chronic renal failure), endocrinopathy (e.g., primary and secondary aldosteronism), and cardiopathy.
Currently, the most commonly used methods to detect serum potassium are ion-selective electrode (ISE) and flame photometry. ISE relies on ion-specific electrodes. Ideally, each electrode possesses a unique ion-selective property that allows it to respond to the desired ion. However, in practice, interference from other ions in the sample compromise the specificity of the detecting electrode, rendering the electrodes susceptible to false readings. Moreover, the instrumentation for ISE is relatively expensive, requires routine maintenance that is sometimes cumbersome and time-consuming, and demands that the operating technician have a considerable degree of skill and knowledge to achieve accurate and consistent readings. Flame photometry relies on the principle that certain atoms, when energized by heat, become excited and emit a light of characteristic wavelength of radiant energy when returning to ground state. The intensity of the characteristic wavelength of radiant energy produced by atoms in the flame is directly proportional to the number of atoms excited in the flames, which is directly proportional to the concentration of the substance of interest in the sample. Like ISE, the instrumentation required for this method is complex and relatively expensive. Moreover, flame photometry requires the use of combustible gas, introducing sometimes expensive hazard prevention measures. Thus, conventional methods to detect potassium ions in samples are limited by complex instrumentation, potentially expensive and cumbersome maintenance, additional hazards, and often time requirements not suitable to emergency situations.
Potassium detection can also be achieved by enzymatic detection. Pyruvate kinase (U.S. Pat. Nos. 5,501,958 and 5,334,507) and glycerol dehydrogenase (U.S. Pat. No. 5,719,036) are described in enzymatic potassium detection methods. However, for each of these enzymes, interference from other ions constitutes a major limitation on the accuracy and usefulness of the assay. For example, the inference from NH3 ions requires additional steps with anti-interference agents to render the enzymatic method accurate.
The present invention addresses the problems with the conventional and enzymatic detection systems discussed above and provides an efficient, inexpensive, and accurate method in a format that is more user friendly in automated analyzers.